Longitudinal Combustion Instability in a Rocket Engine with a Single Coaxial Injector

Longitudinal combustion instability in liquid-propellant rocket engines is investigated using an in-house axisymmetric, multispecies compressible flow solver. Turbulence is treated using a hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation approach. The combustion–turbulence interactions are modeled using the flamelet/progress-variable approach. The computational time is at least an order of magnitude smaller than existing axisymmetric simulations with a more detailed chemistry mechanism. Three flow configurations with a choked nozzle are studied: unstable cases with 12 and 14 cm oxidizer post lengths, and a semiunstable case with a 9 cm oxidizer port length. Numerical results for oscillation frequencies and amplitudes agree with the continuously variable resonance combustor experiments conducted at Purdue University. Good agreements with existing computational results are also found. An additional open-end constant-pressure simulation with (otherwise) the same configuration as the continuously variable resonance combustor experiment with the 14 cm oxidizer post is performed, yielding minor acoustic oscillation. The open-end vortex-shedding frequency is found to be roughly one-half of the frequency of vortex shedding calculated in the continuously variable resonance combustor configuration. The flame region compacts as the combustion chamber becomes more unstable. The choked nozzle allows large-amplitude oscillation, thereby enhancing mixing and leading to more complete combustion near the pressure antinode (combustion chamber entrance) as compared to the open-end chamber. Pulse timing in the unstable case is identified as a major factor for the instability mechanism. Oscillatory behaviors for all three instability domains are described.